1. Field of the Invention
The invention relates to an image sensor, in particular to a CMOS image sensor, and methods for reading out pixels of the image sensor. CMOS image sensors used e.g. in digital cameras require a reading out of pixels with a high rate and a correction of defective pixels.
2. Description of the Prior Art
A common image sensor comprises a plurality of pixels arranged as a pixel array with rows and columns of pixels and a read-out arrangement for reading out the pixels row by row, wherein one column readout circuit is associated with each column of the pixel array. A large number of column readout circuits is needed.
An image sensor, which is used to produce an image representing an object, is described in the EP 1 450 554 A2. The image sensor comprises a pixel array with rows and columns of pixels and a read-out arrangement for reading out the pixels row by row, wherein the M columns, i.e. the M pixels of a row, are grouped into N blocks with M/N pixels. The read-out arrangement comprises a first group and a second group of column readout circuits. Each group has M/N column readout circuits. In operation, the first group of column readout circuits is reading and storing pixel signals from the pixels while the second group of column readout circuits is transferring out processed signal. Two groups of M/N column readout circuits are needed.
In addition, CMOS image sensors used in digital cameras are manufactured with standard processes as provided by leading technology vendors. Due to variances in process parameters, tolerances in mask sets etc. there is a certain probability that not all pixels on a large sensor array perform as specified. Some of them may deliver a permanent signal that corresponds to either dark or bright whereas the latter is the most annoying to viewers on a screen.
To sort out all devices with defective pixels is normally not an option due to the impact on IC costs. So in most cases the customer of an image sensor IC will accept devices from the fab having a certain number of defective pixels as long as the rest of the IC functionality is available as specified. Also during normal lifetime of the product one or more pixels may fail. In return measures have to be taken to compensate for the adverse visible effects of damaged pixels on a screen. Common methods are filtering algorithms that try to adapt the content of the damaged pixel to the surrounding area.
The filtering algorithms are used for the readout data in the digital domain. In U.S. Pat. No. 7,009,644 B1 an intelligent host is provided for the algorithms in order to correct anomalous pixels. Methods described in U.S. Pat. No. 6,665,009 B1 and US 2005/0030394 B1 use additional circuits arranged on the imager chip for the correction.